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Date: Thu, 11 Oct 2012 12:01:37 +0100
From: Mel Gorman <mel@csn.ul.ie>
Subject: Re: [PATCH 04/33] autonuma: define _PAGE_NUMA
Message-ID: <20121011110137.GQ3317@csn.ul.ie>
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To: Andrea Arcangeli <aarcange@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Linus Torvalds <torvalds@linux-foundation.org>, Andrew Morton <akpm@linux-foundation.org>, Peter Zijlstra <pzijlstr@redhat.com>, Ingo Molnar <mingo@elte.hu>, Hugh Dickins <hughd@google.com>, Rik van Riel <riel@redhat.com>, Johannes Weiner <hannes@cmpxchg.org>, Hillf Danton <dhillf@gmail.com>, Andrew Jones <drjones@redhat.com>, Dan Smith <danms@us.ibm.com>, Thomas Gleixner <tglx@linutronix.de>, Paul Turner <pjt@google.com>, Christoph Lameter <cl@linux.com>, Suresh Siddha <suresh.b.siddha@intel.com>, Mike Galbraith <efault@gmx.de>, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, Lai Jiangshan <laijs@cn.fujitsu.com>, Bharata B Rao <bharata.rao@gmail.com>, Lee Schermerhorn <Lee.Schermerhorn@hp.com>, Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>, Alex Shi <alex.shi@intel.com>, Mauricio Faria de Oliveira <mauricfo@linux.vnet.ibm.com>, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>, Don Morris <don.morris@hp.com>, Benjamin Herrenschmidt <benh@kernel.crashing.org>

On Thu, Oct 04, 2012 at 01:50:46AM +0200, Andrea Arcangeli wrote:
> The objective of _PAGE_NUMA is to be able to trigger NUMA hinting page
> faults to identify the per NUMA node working set of the thread at
> runtime.
> 
> Arming the NUMA hinting page fault mechanism works similarly to
> setting up a mprotect(PROT_NONE) virtual range: the present bit is
> cleared at the same time that _PAGE_NUMA is set, so when the fault
> triggers we can identify it as a NUMA hinting page fault.
> 

That implies that there is an atomic update requirement or at least
an ordering requirement -- present bit must be cleared before setting
NUMA bit. No doubt it'll be clear later in the series how this is
accomplished. What you propose seems ok but it all depends how it's
implemented so I'm leaving my ack off this particular patch for now.

-- 
Mel Gorman
SUSE Labs

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